The present invention relates to an array of light-emitting elements and an optical head assembly, more particularly to an array and assembly employing organic electroluminescent (EL) elements.
One well-known application of light-emitting arrays is in the optical heads of electrophotographic printers. Linear arrays of light-emitting diodes (LEDs) have long been applied in this way, for example. The optical head assembly of a typical LED printer comprises a printed circuit board on which are mounted a plurality of LED array devices and their driver integrated circuits (ICs). The diodes in the arrays form an image by illuminating a photosensitive drum.
This LED printing technology is limited in resolution, however, because the LED array devices and their driver ICs are interconnected by wire bonding. A separate bonding wire is required for each LED. Mutual contact between different bonding wires must naturally be avoided, so the LEDs cannot be spaced too closely together. Even if driver ICs are provided on both sides of the LED arrays, so that the bonding wires can be placed alternately on one side and the other, high dot resolution (higher than six hundred dots per inch, for example) appears to be unachievable.
Resolution is not the only problem in the use of LED arrays. Assembly of the optical head is time-consuming and difficult, because even at the comparatively moderate resolution of three hundred dots per inch, printing on standard letter-size (or A4 size) paper requires a row of over two thousand dots, so there are over two thousand bonding wires to be individually attached. Moreover, since each LED array typically provides only sixty-four LEDs, there are some forty LED array devices and a similar number of driver ICs to be individually die-bonded to the printed-circuit board, in precise positional relationships to one another.
Reliability is a further problem: with over two thousand closely-spaced bonding wires, it is difficult to ensure that no pair of adjacent bonding wires will make mutual contact. Cost is also a problem. Fabrication of the LED arrays is expensive, because a compound semiconductor such as gallium arsenide (GaAs) is employed, and the small size of compound semiconductor wafers permits comparatively few arrays to be fabricated per wafer, driving up the cost per array.
Use of arrays of zinc-sulfide (ZnS) light-emitting elements has also been proposed. Zinc sulfide is an electroluminescent material that can be deposited directly onto the surface of a driver IC, eliminating the need for separate array and driver devices. A ZnS light-emitting element, however, is not as easily driven as a LED. The necessary drive voltage is measured in tens of volts and may exceed one hundred volts, so special drive circuitry capable of withstanding high voltages must be provided. Furthermore, an alternating-current power supply must generally be employed, instead of the direct-current supplies normally used with integrated circuits.